A charged particle beam instrument is an apparatus for accelerating particles (charged particles) having an electric charge, such as electrons or positive ions, using an electric field, and irradiating a specimen with the charged particles. The charged particle beam instrument performs observation, analysis, processing and the like of the specimen using an interaction of the specimen and the charged particles. Examples of the charged particle beam instrument include an electron microscope (EM), an electron beam lithography (EB) system, a focused ion beam (FIB) system, and an ion microscope (IM). Among those charged particle beam instruments, the electron microscope is an apparatus for performing observation of a microscopic structure or analysis of a constituent element by irradiating the specimen with electrons and detecting the interaction of the electrons and the specimen as a signal. In the following, the electron microscope will be described.
The electron microscope is often provided with an electron acceleration system using a negative high voltage potential. The electron microscope is provided with an electron source for producing free electrons in a vacuum. The electron microscope produces a flux of a number of electrons having a certain kinetic energy (electron beam) by accelerating the free electrons from the electron source using a potential difference. Most of electron microscopes have a structure such that, in order to obtain the electron beam, the electron source is placed at a cathode having a negative high voltage potential while placing an anode at ground level. The electrons are accelerated to desired energy by the potential difference between the cathode and the anode, producing the electron beam. The part of the electron microscope in which the electron beam is produced is called an electron gun. In many electron microscopes, the energy of the electron beam produced by the electron gun corresponds to the high voltage potential in the electron source portion of the electron gun.
Examples of the electron microscope include a scanning electron microscope (SEM), a scanning transmission electron microscope (STEM), and a transmission electron microscope (TEM). The scanning electron microscope is an apparatus that utilizes secondary electrons or back scattered electrons and the like from the specimen surface, and requires a maximum electron beam energy of approximately 30 kV.
The scanning transmission electron microscope and the transmission electron microscope are apparatuses that observe electrons transmitted through or scattered in the specimen, and typically require electron beam energy of 100 kV or more. As mentioned above, many of the electron microscopes have the structure where the electron beam energy is provided by a high voltage potential in the electron source portion. Thus, in an electron microscope that requires higher electron beam energy, it is necessary to insulate the high voltage potential of the electron source portion from the surrounding ground level portions so that the potential can be stably maintained.
Accordingly, in the electron gun having an acceleration voltage in excess of 100 kV, an accelerating tube structure is often adopted. In the accelerating tube structure, a structure (accelerating tube) comprising insulator tubes and metal rings alternately stacked one upon another is assembled between the electron source portion (cathode) which is maintained at a negative high voltage potential, and the anode which is an electron beam exit and which is at ground level. The accelerating tube structure is adopted, instead of directly connecting the electron source portion and the anode using a single insulator tube, for the following reasons. When there is a potential difference between the ends of a long insulator, instead of the contour lines of electric potential being smoothly distributed over the insulator between the electrodes on both ends, a portion with narrow intervals of the contour lines of electric potential (electric field concentrated portion) is created at a portion closer to one of the ends of the long insulator. In the electric field concentrated portion, electrical breakdown may be caused upon application of a high voltage. Thus, in the accelerating tube structure, a stacked structure of insulator tubes and interspace electrodes is adopted, and further the interspace electrodes are compulsorily given intermediate potentials between the electron source and the ground level using bleeder resistors. As a result, the insulator tubes only need to provide the function of insulating between the electrodes on both sides of the insulator tubes, whereby the high potential portion can be stably maintained.
Insulation of the accelerating tube structure will be considered. Between the accelerating tube and the surrounding ground level portions, there is a potential difference of a maximum of 200 kV, for example. The potential difference may cause a discharge in the space between the high potential portion of the accelerating tube and the ground level portion of a container. Thus, the electron gun requires the function of stably maintaining the potential difference (insulator portion).
Patent Literature 1 discloses a structure where the accelerating tube is surrounded by a metal container. In Patent Literature 1, the metal container is disposed around the accelerating tube and is set at ground level. In Patent Literature 1, in order to prevent discharge in the space between the high potential portion of the accelerating tube and the ground level portion of the container, the container is filled with gas insulator or liquid insulator. In this way, a discharge between electrodes having a potential difference is prevented.